The Nature of Lower Mantle Seismic Anomalies

Lateral variation of temperature is the dominant influence on seismic anomalies in the upper mantle through contributions from the intrinsic temperature derivatives, velocity dispersion as a result of anelasticity, and phase equilibria. Although intrinsic temperature derivatives are suppressed at higher pressures, high velocitiy anomalies are expected to persist within the cold cores of slabs subducted into the lower mantle. The apparent general failure of this expectation has fueled continued speculation for over a decade concerning the fate of subducted slabs and the nature of mantle circulation. The discovery of a high-spin to low spin transition in iron-bearing lower mantle phases provides a mechanism to further suppress intrinsic temperature derivatives of elasticity. Key here is the thermodynamic nature of spin transitions and its connection to elasticity. In a pressure regime of 40 to 60 GPa all three elastic constants of ferropericlase smoothly pass through minima with respect to an extrapolation of the pure high-spin state. This behavior is consistent with a macroscopic thermodynamic description that predicts no sharp changes in spin fractions and dependent physical properties. A significant consequence is that in the spin transition pressure regime, normal temperature derivatives of elasticity are reduced. Within Earth, a minimum in the temperature derivatives caused by the transition is predicted to occur at a depth of 1500 km. This correlates well with the depth where seismic tomographic results show minimum structure and model power. Thus, the spin transition may serve to mask or even invert velocity anomalies associated with lateral temperature differences.